Wind turbines are occasionally exposed to over-voltage from the power grid side due to power grid instabilities. Usually, if the over-voltages exceed predetermined values, the wind turbines become disconnected from the grid to avoid any damage to the wind turbine system. Disconnecting a wind turbine from a power grid, however, may generate significant down times since it takes some time to reconnect the wind turbine to the grid again. Even more, disconnecting wind turbines from the power grid in case of over-voltages due to power grid instabilities may contribute additional instability to the power grid. Disconnecting a wind turbine from the power grid in case of grid instability therefore may lead to a chain reaction that severely damages grid stability.
In order to overcome the danger of grid instability and wind turbine damage, grid utility providers usually request minimum over-voltage tolerances that wind turbines have to withstand before they are allowed to disconnect from the power grid. This has led to the effect that at present, components of current wind turbines that provide a voltage of 690 V at 50 Hz are designed to withstand an over-voltage of 110% for all times, an over-voltage of 115% for a duration of 1 s, and an over-voltage of 120% for a duration of 100 ms. However, in the meantime, countries or regions like Scottland have established grid over-voltage requirements that require a wind turbine to ride through a power over-voltage of up to 150%. Such high over-voltages are likely to damage present wind turbines.